I was initially thinking of doing something like getting the subs in vermont maple and the speakers in Russian Maple since they have a bit of a dark grain running through them. It would give a bit of contrast but it's really hard to tell how that would look.

Yes, I've seen the Burnt Sienna grilles back at the Axiom plant. As Ken C noted, they do have an orangey tinge but "rust-colored" is how I'd describe them. They do provide a nice contrast with some of the finishes, however actually getting samples is really the only way to absolutely know.